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Engineering Controls Database

Guidelines for the Control and Monitoring of Methane Gas on Continuous Mining Operations – Methane Monitoring – Methanometers, Catalytic Heat and Combustion Sensors

The introduction of conventional mining methods, which increased the rate of mining, was an important step in the mechanization of mining. The intermittent nature of the conventional mining process halted the extraction process for ore-loading and usually allowed time for methane gas to be dispersed. However, the introduction of continuous mining machines in the 1940s produced a constant flow of ore from the working face of the mine and resulted in an increase in methane levels.

The number of face ignitions increased as more continuous mining machines were placed underground. Methane levels were found to be dangerously high. In some cases, methane concentrations measured 20 ft from the mining face exceeded the lower explosive limit (5% by volume) [USBM 1958]. The need for better face area ventilation was recognized to reduce the potential for explosions.
Excessive levels of methane gas can affect the safety of the underground work force. Available methane control systems have been challenged in recent years by mining developments which include the use of continuous mining machines.

Most mining accidents today generally involve only a few individuals. However, the infrequent occurrence of gas explosions puts the lives of the entire underground workforce at risk. In the past 10 years, explosions have led to 65 fatalities and 18 injuries with major explosions occurring at the Sago Mine in West Virginia in 2006 (12 fatalities and 1 injury), the Darby No. 1 Mine in Kentucky in 2006 (5 fatalities and 1 injury) and, most recently, at the Upper Big Branch Mine in West Virginia in 2010 (29 fatalities) [NIOSH 2011].
The U.S. Bureau of Mines (USBM) was formed in 1910 following a series of underground explosions that resulted in many fatalities and injuries [Kirk 1996]. The agency was responsible for conducting scientific research and disseminating information on the extraction, processing, use, and conservation of mineral resources. The USBM research program for mining health and safety was transferred to NIOSH in 1996. Since that time, NIOSH has established a ventilation test gallery where techniques for methane control and monitoring are evaluated under a variety of conditions that simulate airflow near the working face of a continuous mining section. Airflow patterns and methane concentrations are studied in a detailed manner that is not possible in a working underground mine.

Effect of Water Sprays on Face Airflow and Methane

Engineering controls such as water sprays and scrubbers help maintain safe methane levels in coal mines. Methane monitoring is required to ensure that engineering controls are effective and that methane concentrations do not exceed regulatory standards. NIOSH research examined instruments and sampling strategies used for methane sampling and developed methods for evaluating performance. The research included monitors using both catalytic heat of combustion and infrared absorption sensors.

Methanometers, Catalytic Heat and Combustion Sensors

Figure 1 shows a catalytic heat of combustion sensor with its sensor elements. The catalytic sensor includes a very fine platinum wire contained within an alumina bead coated with a catalyst material, typically platinum or palladium. During operation, the bead (active) is heated to a temperature sufficient to promote combustion of the methane gas with oxygen on the surface of the catalyst. The increased heat generated by the combustion increases the resistance of the wire inside the bead. The change in resistance is monitored with a Wheatstone bridge circuit, which generates an electrical signal proportional to the methane concentrations. A second bead (inactive) in the bridge, not treated with the catalyst material, is used to compensate for changes in temperature, pressure, and humidity.

The sensor head must be covered with a dust cap to protect the sensor element or sensor chamber from dust or water [30 CFR 27.22]. Within the cap, a flame arrestor made of screen or porous material is used to prevent any ignition of methane gas from moving outside the sensor head.
Figure - 1 - Sampling head with catalytic heat of combustion sensor.

Figure - 1 - Sampling head with catalytic heat of combustion sensor.

NOTE: The above control information is taken directly from the following publication:
NIOSH [2010]. Information circular 9523. Guidelines for the control and monitoring of methane gas in continuous mining operations. Morgantown, WV: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2010-141.
CFR. Code of Federal Regulations. Washington, DC: U.S. Government Printing Office, Office of the Federal Register.

Kirk WS [1996]. The history of the Bureau of Mines. In: U.S. Bureau of Mines Minerals Yearbook, 1994. Washington, DC: U.S. Bureau of Mines.

NIOSH [2011]. Ventilation and explosion prevention highlights.

USBM [1958]. Auxiliary ventilation of continuous miner places. By Stahl RW. Washington, DC: U.S. Bureau of Mines, Report of Investigations, No. 5414.
coal mining
continuous mining operations
deep-cut mining